Diagnosis and treatment of fully or partially blocked arteries of the heart is essential in the medical profession's endeavor to prevent heart attacks. Physicians have successfully prevented heart attacks arising from artery blockage caused by the build-up of plaque upon the walls of the coronary arteries through the use of percutaneous transluminal coronary angioplasty (PTCA, commonly referred to as "balloon angioplasty"). Balloon angioplasty involves carefully threading a catheter into the affected portion of the artery. After the balloon portion is determined to be properly positioned in the artery, the physician inflates the expandable portion of the catheter in order to broaden the blocked or narrowed passage in the blood vessel caused by the deposition of plaque upon the artery wall.
The desirability of using an imaging device to produce treatment and diagnostic quality images of small enclosed areas such as human blood vessels on a diagnostic video display device is unquestioned. It is known to use a very small ultrasonic imaging device mounted at the end of a catheter to produce a real-time image of the internal walls of a coronary artery. This device is referred to herein as an ultrasound catheter.
In the known ultrasound catheters, the same material is used for the electronics carrier upon which a set of electronic components are mounted and for the backing material for the transducer assembly. A drawback to the known ultrasound catheters is the difficulty in finding a carrier/backing material which provides the physical and acoustic qualities desired for advantageous use as the carrier for the electronics and the backing material for a transducer assembly comprising a highly sensitive transducer material.
The known ultrasonic catheter structure, though providing the advantage of design and construction simplicity, exhibits certain drawbacks attributable to the particular and mutually incompatible requirements for the backing material and the electronics carrier. It is desirable that the electronics carrier for the electronics body be rigid and capable of withstanding the elevated temperatures produced by the electronics. However, the known electronics carrier materials which satisfy the requirements for the electronics body are not suitable backing materials for the presently preferred transducer assemblies comprising highly sensitive lead zirconate titanate (PZT) composites.
When the new, more sensitive PZT composites are used with the known electronic carrier material as the backing material for the transducer, unwanted ringing occurs in the transducer assembly when an acoustic signal is received or transmitted by the catheter. The signal produced by the ringing reduces the quality of the signal transmitted by the transducer assembly and limits the foreseeable advantages of utilizing the more sensitive transducer materials in ultrasonic catheters. The decreased signal quality attributed to the ringing limits the image quality provided by an ultrasound catheter. The limited image quality restricts the usefulness of the ultrasound catheter for clinical and diagnostic imaging.
In known ultrasound catheters the transducer electrodes are coupled to the transducer layer through a capacitive glue layer. As was previously mentioned, PZT composites having a relatively high degree of sensitivity to acoustic signals are being considered for replacement of the previously used, less sensitive, ferroelectric polymer transducer materials. While the PZT composites exhibit superior sensitivity in comparison to the ferroelectric copolymers, they also have a higher dielectric constant. The reduced impedance (or increased capacitance) associated with the new PZT composites significantly negates the improved signal sensitivity provided by the PZT composites when coupled to the transducer electrodes through the capacitive glue layer.